1. Field of the Invention
The present invention is generally in the field of electrical circuits. More specifically, the invention is in the field of power amplifiers.
2. Related Art
Power amplifiers used in the transmitter section of communications devices, such a wireless handsets, are required to operate under a wide variety of operating conditions, such as variations in temperature and supply voltage, and variations in load impedance. Under these varying operating conditions and load impedance, it is highly desirable for power amplifiers to maintain a constant output power. As a result, various techniques have been employed in an attempt to control the output power of a power amplifier.
However, current techniques for controlling the output power of a power amplifier suffer various disadvantages. For example, techniques that indirectly control the output power by controlling the voltage or current, such as collector voltage or collector current, supplied to the power amplifier provide adequate compensation for variations in operating conditions, but undesirably allow significant variations in power delivered to the load. For example, in the voltage or current control techniques discussed above, the power delivered to the load by the power amplifier can vary by as much as 10.0 decibels (dB) as the impedance of the load changes.
In another conventional approach, a coupler having directivity greater than approximately 10.0 dB is implemented to couple to and sense the output power delivered to the load. However, this approach requires radio frequency (RF) circuitry for detecting and controlling the output power of the power amplifier, which increases cost and circuit design complexity. Additionally, this approach can also cause undesirable frequency variations in the RF output signal generated by the power amplifier, which require system level calibration for correction.
Thus, there is a need in the art for a cost-effective power control circuit that accurately controls the output power of a power amplifier.